Update doc/user-guide/src/concepts/toolchains.md

Co-authored-by: rami3l <[email protected]>

Author: kyle-leonhard

doc/user-guide/src/concepts/toolchains.md

@@ -17,7 +17,7 @@ Standard release channel toolchain names have the following form:
 
 <channel>       = stable|beta|nightly|<versioned>[-<beta tag>]
 <versioned>     = <major.minor>|<major.minor.patch>
-<beta tag>      = beta[.<number>]
+<prerelease>      = beta[.<number>]
 <date>          = YYYY-MM-DD
 <host>          = <target-triple>
 ```